Autoxidation of conjugated linoleic acid methyl ester in the presence of α-tocopherol: the hydroperoxide pathway

The autoxidation of conjugated linoleic acid (CLA) is poorly understood in spite of increasing interest in the beneficial biological properties of CLA and growing consumption of CLA-rich foods. In this thesis, the autoxidation reactions of the two major CLA isomers, 9-cis,11-trans-octadecadienoic acid and 10-trans,12-cis-octadecadienoic acid, are investigated. The results contribute to an understanding of the early stages of the autoxidation of CLA methyl ester, and provide for the first time a means of producing and separating intact CLA methyl ester hydroperoxides as well as basic knowledge on lipid hydroperoxides and their hydroxy derivatives. Conjugated diene allylic monohydroperoxides were discovered as primary autoxidation products formed during autoxidation of CLA methyl esters in the presence and absence of α-tocopherol. This established that one of the autoxidation pathways of CLA methyl ester is the hydroperoxide pathway. Hydroperoxides were produced from the two major CLA methyl esters by taking advantage of the effect of α-tocopherol to promote hydroperoxide formation. The hydroperoxides were analysed and separated first as methyl hydroxyoctadecadienoates and then as intact hydroperoxides by HPLC. The isolated products were characterized by UV, GC-MS, and NMR techniques. In the presence of a high amount of α-tocopherol, the autoxidation of CLA methyl ester yields six kinetically-controlled conjugated diene monohydroperoxides and is diastereoselective in favour of one particular geometric isomer as a pair of enantiomers. The primary autoxidation products produced from the two major CLA isomers include new positional isomers of conjugated diene monohydroperoxides, the 8-, 10-, 12-, and 14-hydroperoxyoctadecadienoates. Furthermore, two of these new positional isomers have an unusual structure for a cis,trans lipid hydroperoxide where the allylic methine carbon is adjacent to the cis instead of the usual trans double bond. The 1H and 13C NMR spectra of nine isomeric methyl hydroxyoctadecadienoates and of ten isomeric methyl hydroperoxyoctadecadienoates including the unusual cis,trans hydroperoxides, i.e. Me 8-OOH-9c,11t and Me 14-OOH-10t,12c, were fully assigned with the aid of 2D NMR spectroscopy. The assigned NMR data enabled determination of the effects of the hydroxyl and hydroperoxyl groups on the carbon chemical shifts of CLA isomers, identification of diagnostic signals, and determination of chemical shift differences of the olefinic resonances that may help with the assignment of structure to as yet unknown lipid hydroperoxides either as hydroxy derivatives or as intact hydroperoxides. A mechanism for the hydroperoxide pathway of CLA autoxidation in the presence of a high amount of α-tocopherol was proposed based on the characterized primary products, their relative distribution, and theoretical calculations. This is an important step forward in CLA research, where exact mechanisms for the autoxidation of CLA have not been presented before. Knowledge of these hydroperoxide formation steps is of crucial importance for understanding the subsequent steps and the different pathways of the autoxidation of CLA. Moreover, a deeper understanding of the autoxidation mechanisms is required for ensuring the safety of CLA-rich foods. Knowledge of CLA oxidation and how it differs from the oxidation of nonconjugated polyunsaturated fatty acids may also be the key to understanding the biological mechanisms of CLA activity.

Late Transition Metal and Aluminum Complexes for the Polymerization of Ethene and Acrylates

Polyethene, polyacrylates and polymethyl acrylates are versatile materials that find wide variety of applications in several areas. Therefore, polymerization of ethene, acrylates and methacrylates has achieved a lot attention during past years. Numbers of metal catalysts have been introduced in order to control the polymerization and to produce tailored polymer structures. Herein an overview on the possible polymerization pathways for ethene, acrylates and methacrylates is presented. In this thesis iron(II) and cobalt(II) complexes bearing tri- and tetradentate nitrogen ligands were synthesized and studied in the polymerization of tertbutyl acrylate (tBA) and methyl methacrylate (MMA). Complexes are activated with methylaluminoxane (MAO) before they form active combinations for polymerization reactions. The effect of reaction conditions, i.e. monomer concentration, reaction time, temperature, MAO to metal ratio, on activity and polymer properties were investigated. The described polymerization system enables mild reaction conditions, the possibility to tailor molar mass of the produced polymers and provides good control over the polymerization. Moreover, the polymerization of MMA in the presence of iron(II) complex with tetradentate nitrogen ligands under conditions of atom transfer radical polymerization (ATRP) was studied. Several manganese(II) complexes were studied in the ethene polymerization with combinatorial methods and new active catalysts were found. These complexes were also studied in acrylate and methacrylate polymerizations after MAO activation and converted into the corresponding alkyl (methyl or benzyl) derivatives. Combinatorial methods were introduced to discover aluminum alkyl complexes for the polymerization of acrylates and methacrylates. Various combinations of aluminum alkyls and ligands, including phosphines, salicylaldimines and nitrogen donor ligands, were prepared in situ and utilized to initiate the polymerization of tBA. Phosphine ligands were found to be the most active and the polymerization MMA was studied with these active combinations. In addition, a plausible polymerization mechanism for MMA based on ESI-MS, 1H and 13C NMR is proposed.